Method for maskless chemical and electrochemical machining

ABSTRACT

A method for high resolution maskless chemical and electrochemical machining is described. Preferential etching results from exposing those regions where machining is sought to an energy beam. Such exposures can increase the ething rate in the case of electrochemical machining by a factor of 10 3  to 10 4 . Such enhancement is sufficient to make masking unnecessary.

DESCRIPTION

1. Technical Field

This invention relates generally to a method for enhancing the rate ofchemical and electrochemical machining, more particularly to a masklesstechnique employing an energy beam to establish high resolution chemicaland electrochemical machined patterns. Copending applications 037,075now U.S. Pat. No. 4,217,183 and 037,073 both of which were filed May 8,1979, respectively treat methods for locally enhancing the rate ofelectroplating, and for locally enhancing the rate of electrolessplating.

2. Background Art

The classical methods of machining which rely on deformation mechanismsare limited in their resolution and result in deformation of themachined surface. The problems associated with deformation of themachined surface have in part been overcome by electrochemical machiningtechniques. W. Kern and J. M. Shaw, in Journal of ElectrochemicalSociety, Electrochemical Technology, Vol. 118, No. 10, pp. 1699-1704disclose a technique for delineation of high resolution patterns intungsten films on semiconductor device wafers. Their process is based onhigh speed anodic dissolution of tungsten. In order for the dissolutionto be preferential, it is necessary to employ a photoresist mask whichis applied before machining the surface. The photoresist is selectivelyexposed to develop a pattern, the exposed surface is electrochemicallymachined, and finally the mask is stripped. This multistep process istime consuming and costly.

Maskless techniques for electrochemical machining are available. Thesetechniques employ a shaped cathode brought in close proximity to theworkpiece, which serves as the anode. A flowing salt solution serves asthe electrolyte between the cathode and the workpiece. As a voltage isapplied to make the workpiece anodic with respect to the cathode, metalions from the workpiece are preferentially transferred to the liquid inthe vicinity of the cathode. As the contour of the workpiece changes,the cathode is advanced to maintain its proximity to the workpiece. Thistechnique is suited for sinking shallow axially symmetric cavities intoa surface, but is limited with respect to the configurations that may begenerated or to the dimensional lower limits which can be achieved. Dueto the need of flowing the electrolyte at high speeds past the surfaceto carry away the insoluble products of reaction and to cool the partand the solution high pressure, high capacity pumps are an essentialpart of the equipment. Maskless electrochemical machining techniques arefurther described by J. F. Thorpe and R. D. Zerkle, in an articleentitled "A Theoretical Analysis of the Equilibrium Sinking of Shallow,Axially Symmetric Cavities by Electrochemical Machining", and by W.Konig and H. Degenhardt in an article entitled "The Influence of ProcessParameters and Tool-Electrode Geometry on the Development of the Overcutin Electrochemical Machining with High Current Densities". Both articlesare published in Fundamentals of Electrochemical Machining, (1971)Editor C. L. Faust, Electrochemical Society, Princeton, New Jersey.

A jet stream technique discussed by J. L. Bestel, J. K. Dorey, II, D. J.Fineberg, R. Haynes, K. Ramachandran, R. E. Sinitski and V. Srinivasanin Electrochemical Society, Abstract, 286, Vol. 77, No. 2, page 759(1977), offers an alternative method for maskless plating which wouldalso be applicable to maskless electrochemical machining. To machine, astream or jet of electrolyte impinges on the workpiece, and the streamis maintained at a lower electrical potential than the workpiece. Sincethe workpiece is anodic with respect to the stream, metal ions will betransferred to the stream from the workpiece resulting in selectiveetching. Gerald C. Oliver in an article entitled "Plating Fine Lineswith a Nozzle", in Insulation/Circuits, p. 23-4 (July 1978), points outthat jet plating and etching will allow one to obtain resolution of 100microns or less with present nozzles and that with nozzle openings inthe micron range a line pattern of resolution approaching the dimensionof the nozzle openings is achievable. While Oliver suggests that it maybe possible to plate patterns in the micron range, it should be pointedout that employing micron diameter nozzles for plating or etching willincrease the frequency of the clogging of the nozzles and can result inthe streams which exist from these nozzles being discontinuous.

Laser machining will produce fine holes. Since laser machining isaccomplished by either melting or vaporizing the material removed, thesubstrate will be locally heated. This heating can cause changes in themicrostructure and introduce residual stresses and cracks in theresulting machined surface. The melting or vaporization can also reduceedge definition of the machined holes. These problems associated withlaser machining are further discussed in an article entitled, "Effect ofPulsed Laser Radiation on Thin Aluminum Foils" by M. O. Aboelfotoh andR. J. von Gutfeld, in the Journal of Applied Physics, Vol. 43, No. 9,pages 3789-94 (1972).

Light activated plating which is the converse process to electrochemicalmachining has been employed to produce arbitrary patterns by platingonto a photoconductor exposed to light. C. S. Roberts in U.S. Pat. No.3,013,955 teaches exposing doped regions of silicon to light producing aphotovoltaic effect which promotes plating in the doped regions exposedto light.

P. F. Schmidt in U.S. Pat. No. 3,345,274 and P. F. Schmidt et al in U.S.Pat. No. 3,345,275 teaches the anodization of a photoconductor substrateby exposing to light those areas which are to be anodized.

Similarly G. Suzuki and T. Tomotsu in U.S. Pat. No. 3,935,117 describe aphotosensitive compound which can be decomposed by light to form etchingsolutions.

While the Roberts, Schmidt and the Schmidt et al techniques teachmethods of forming preferential patterns on substrates by exposing thesubstrate or portions thereof to light, these techniques are limited tophotoconducting substrates and do not teach enhancement of the platingrate as a function of the strength of the light source. The samelimitation applies to the etching technique of Suzuki et al with respectto the etchant that can be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of the invention which employs anenergy beam to assist in chemical machining.

FIG. 2 illustrates a second embodiment of the invention which employs anenergy beam to assist in electrochemical machining.

DISCLOSURE OF INVENTION

An object of the present invention is to establish a method for masklesschemical and electrochemical machining. Another object of this inventionis to provide a method for machining that is suitable forpersonalization of integrated circuits and circuit boards. Still anotherobject of this invention is to provide a method for machining contoursor arbitrary shapes. Various other objects and advantages of the presentinvention will become apparent to those skilled in the art from thefollowing description and suggested industrial applications.

The practice of the present invention allows one to chemically machine asurface without masking. The surface to be machined is contacted with anetching solution. An energy source is focused onto the workpiece whereit locally heats the surface to be machined. This process locally speedsup the chemical reaction and enhances the etching rate andpreferentially removes material from the heated surface.

Alternatively, the present invention may be practiced by contacting thesurface to be machined with an electrolyte in which a cathode is placed.Again an energy source is focused onto the workpiece where it locallyheats the surface to be machined. An electric potential is establishedbetween the surface of the workpiece, serving as the anode, and thecathode.

BEST MODES FOR CARRYING OUT THE INVENTION

There are two modes for practicing the present invention: energy beamassisted chemical machining, and energy beam assisted electrochemicalmachining.

The practice of the first mode, energy beam assisted chemical machining,will be described in terms of FIG. 1. There is shown in FIG. 1 a vessel10 which contains an etching solution 12. The workpiece 14 is immersedin the etching solution 12 so as to cause the surface 16 which is to bechemically machined to be contacted by the electrolyte 12. While theworkpiece 14 has been illustrated with the machined surface 16 beinghorizontal, the surface 16 may be placed in a vertical position or anyother convenient position. The etching solution may be either acidic oralkaline. However, in either case an etching solution should be chosenwhich at room temperature exhibits only a very slow etching rate on theworkpiece and which attacks the surface 16 of the workpiece 14 veryuniformly, i.e. not preferred intergranular attack.

Typically for metals such as Ni, Cu, Al, an acid etching solution suchas NHO₃, HCl, and H₃ PO₄ would be appropriate. When the workpiece 14 issubmerged in the solution 12 etching will occur at a nominal orbackground rate on all exposed faces including the surface 16. An energysource 18 is focused by a lens system 20 to concentrate the energy inthe beam 22 which passes through the etching solution 12 and impinges onthe surface 16. An energy source 18 which generates electromagneticradiation in the visible or infrared spectrum is particularly usefulsince a wavelength can be selected so as not to be strongly absorbed bythe etching solution 12 and yet still be strongly absorbed by theworkpiece 14. When it is desired to machine holes which are not greaterthan 100μ in diameter through the entire thickness of the workpiece 14,it is preferred to maintain the workpiece's thickness to approximately 5mil. or less. This will assure mixing of the etching solution in themachined recesses with the bulk solution.

This technique of chemical machining is particularly well suited for aworkpiece 14 which is a composite, in which the machined surface 16 is astrongly optical absorbing material such as a metal, and the substrate17 is a material with a low thermal conductivity such as glass. Thisconfiguration of the workpiece 14 is illustrated in FIG. 2. Thiscomposite structure will enhance the localization of the heat effectedzone and thereby increase the preferential etching in the heatedregions. Again, for the reasons discussed above the metallized surface16 should be thin, preferably less than 5 mil when holes having adiameter of less than 100μ are desired.

The beam 22 exposes the region 24 locally heating that portion of thesurface 16 where machining is sought. The region 24 exposed and heatedby the beam 22 may be so manipulated by moving the workpiece 14 in the xand the y directions as shown in FIG. 1. Alternatively, it is possibleto maintain the workpiece 14 stationary and manipulate the beam 22.

When electromagnetic radiation in the visible range is employed it canbe generated from a carbon arc but preferably a laser light source suchas a continuous light from a multimode argon laser or krypton tunablelaser is desired. The laser provides a high intensity energy source 18which may eliminate the necessity of a lens system 20 unless furtherfocusing is desired to reduce the beam size 22. The intensity of thelight in all cases should be sufficient to provide a beam 22 with anintensity preferably between about 10² to 10⁶ W/cm².

The lower limit is required to provide sufficient heating forenhancement of the etching rate, while the upper limit should be chosento avoid thermotransformation of the structure of the work piece. Ingeneral, this will limit the maximum power input to about 10⁶ W/cm².When a beam 22 passes through the etching solution 12 care must be takenin selecting the wavelength of the light to avoid a frequency that isstrongly absorbed by the etching solution 12.

Alternative etching arrangements are available where the beam 22 neednot pass through the etching solution 12. If for example, only thesurface 16 of the workpiece 14 is brought in contact with the etchingsolution 12, the beam 22 may expose the workpiece 14 on the sideopposite the surface 16, which is where machining is sought. When theworkpiece 14 in the vicinity to be machined is not greater than about 5mil in thickness it is possible to generate sharp temperature profilesthrough the workpiece 14, and thereby establish temperature contours inthe surface 16 similar to the contours that would result from passingthe beam 22 through the etching solution 12 and directly expose thesurface 16.

The same technique can be used for a thicker workpiece 14 which has astrongly absorbing surface 16 which typically could be a metal and asubstrate 17 which is transparent to the radiation. In this case, it ispreferred that the metallized coating be generally less than 5 mil. forholes less than approximately 100μ.

The beam 22 emitted from the energy source 18 may be modulated by amodulator 26 which may be placed between the energy source 18 and thelens system 20, or alternatively between the lens system 20 and theworkpiece 14. The modulator 26 may be a mechanical light chopper whenthe modulation rate is low or an optical modulator can be employed whenmore rapid modulation is sought. Optical modulation will allowfrequencies up to gigahertz.

Preferential etching will occur in the region 24 heated by the impinginglight. Modulating or pulsing of the light provides sharper temperatureprofiles in the vicinity of the light exposed region 24, and enhancesthe etching rate and improves the edge definition.

Modulation of the laser light has the effect of limiting the thermalspread which occurs in the substrate through thermal conduction andcauses a decrease in the resolution.

The practice of the second mode, electrochemical machining, will bedescribed in reference to FIG. 2. There is shown in FIG. 2 a vessel 10which contains an electrolyte 12.

Typically for a metal such as Cr, Ni, or Ti an electrolyte such as NaClsolution would be appropriate.

The workpiece 14 is immersed in the electrolyte 12. The workpiece 14 forthis mode must be conducting and may be a metal, photoconductor or acomposite structure of an insulator and an electrical conductor. When acomposite structure is employed having an electrically nonconductingsubstrate it is preferred that the thickness of the metallized surface16 be less than 5 mil. A cathode 30 is placed in the electrolyte 12 andis spaced apart from the workpiece 14, which serves as an anode. Anenergy source 18 is focussed by a lens system 20 to form a beam 22 whichpasses through an opening 31 in the cathode 30 and exposes the region 24of the surface to be machined 16. When a laser is employed as the energysource 18 the lens system 20 may be optional. As heretofore describedfor the first mode the beam 22 may be manipulated with respect to themachined surface 16 by movement of the workpiece 14 as illustrated inFIG. 1 or alternatively the beam 22 may be manipulated via a scanningmirror 32 allowing a predetermined portion of the surface 16 of theworkpiece 14 to be exposed.

Alternative etching schemes are available where the beam 22 is notpassed through the electrolyte 12. These schemes in all respects aresimilar to those earlier discussed for chemical machining. A voltagesource 34 is connected between the workpiece 14 and the cathode 30,which is maintained at negative potential with respect to the workpiece14. Again, the beam 22 may be modulated by a voltage modulator 36 tosharpen the temperature profile in the light exposed region 24 andenhance the resolution of the machining. A voltage modulator 36 may beemployed to synchronize the application of the electrical potential tothe modulated light. Synchronization of the laser beam 22 and theetching voltage has the advantage that etching is allowed to occur onlywhen the laser has locally heated the substrate to produce the optimaltemperature gradient. At other times during the modulation cycle theetching voltage is turned off thus reducing the background etching.

When the voltage is applied as described above metallic ions pass intothe electrolyte 12 from the workpiece 14. While there will be a generaltransport of ions from the workpiece 14 into the electrolyte,preferential solutioning of the ions will occur in the region 24 heatedby the impinging light. By appropriately pulsing the light and pulsingthe voltage in synchronism it is possible to obtain etching rates ofapproximately 10⁴ times greater than the background rate, thus negatingthe requirement of masking in order to establish etched patterns in thesurface of a substrate.

While all modes of the present invention have been described in terms ofa single beam 22 impinging on the workpiece 14 one could employ multiplebeams and simultaneously etch at multiple locations.

The following specific examples of the invention give details and are byway of illustration and not by way of limitation.

EXAMPLE I

A 304 stainless steel workpiece and a Pt cathode are placed in a nickelchloride etching solution. The solution has the following composition:

21 gm NiCl₂.6H₂ O

25 gm H₃ BO₃

1.64 gm Na-Saccharine

1.0 gm 2 butyne 1-4 diol

1 l H₂ O

The surface of the workpiece is adjacent to, but separated from thecathode and has an area of approximately 0.3 cm². The anode and thecathode are maintained at constant potential difference. A currentbetween the cathode and the anode of ˜1 ma is established which yields acurrent density of 3.3 ma/cm². A focussed argon laser is employed togenerate a beam having an energy density of 3×10⁵ W/cm² which exposes acircular region of the surface approximately 50μ in diameter. The lengthpulse is 20 ms. The time between pulses is 60 ms. During the laser pulsethe current increases to ˜2 ma. The laser enhanced current densitynormalized over the exposed circular region is 5.0×10⁴ ma/cm². As can beseen the laser associated increase in the current, which is directlyrelated to the etching rate, is a factor of approximately 1.5×10⁴.

EXAMPLE II

When 304 stainless steel stock 2 mil thick is placed in the electrolyteand exposed as set forth in Example I, holes which passed through thestock are produced when a potential difference between the anode andcathode of 1.5 V is applied. In a total elapsed time of approximately 9sec, holes having sharp edge definition and diameter between 25 & 50μare produced. These holes are produced without otherwise damaging thesteel stock.

EXAMPLE III

A 304 stainless steel stock which in this case is 20 mil in thickness isplaced in the electrolyte of Examples I and II. Again the same potentialis applied as for the examples above. An argon laser is employed. Thebeam is focused to 150μ diameter giving a beam density of about 2×10³W/cm². Light enhanced etching of holes was conducted for various periodsof continuous light exposure and the data are tabulated below.

    ______________________________________                                        Time of exposure                                                                           Depth of hole                                                                              ˜ diameter of hole                            ______________________________________                                        2.5          4.8μ      160μ                                             5.0          8.0μ      160μ                                             10.0         16.0μ     175μ                                             15.0         18.3μ     175μ                                             ______________________________________                                    

It can be seen that by increasing the exposure time, one increases themachining depth.

EXAMPLE IV

A 304 stainless steel sample having the geometry of Example III isexposed to a beam as described in Example III. Again a potential of 1.5V is applied between the anode and cathode. In this case, the beam ispulsed with the light being on for 44 ms and off for 130 ms a holehaving a depth of 18μ, a diameter of 115μ is etched. Comparing ExamplesIII & IV one can see that by pulsing the beam the resolution of theetching can be improved.

EXAMPLE V

Workpiece is glass with a 1000 A Ni film deposited on its surface. Theworkpiece is placed in a solution consisting of:

20 gm NaCl

2.5 ml concentrated H₂ SO₄

125 ml H₂ O

The Ni film is maintained using ˜700 mW of light from a krypton laserturned to 6471 A, the incident light is focussed on 16 to a spot ˜170μin diameter (3×10³ W/cm²). Holes in the film are produced where light isallowed to impinge for times of the order of 1-5 sec. a thinning of thefilm to approximately half its thickness is observed along the beam pathwhen the film is moved in the Y direction at a rate ˜2 mm/sec. When theacid solution is replaced by water and the experiment is repeated, noeffect on the film is observed.

INDUSTRIAL APPLICABILITY

The above described methods for chemical machining and electrochemicalmachining are well suited to application where maskless high resolutionchemical and electrochemical machining is desired. The technique is wellsuited for hole drilling, personalization and repair of integratedcircuits. The technique can also be used for circuit formation byetching particularly in which the area of metal removed is smaller thanthat of the metal remaining behind. The techniques described should beuseful in the electronics packaging and related industries.

While the novel features of the invention have been described in termsof preferred embodiments and for particular industrial applications, itwill be understood that the various omissions and substitutions in theform and details of the method described may be made by those skilled inthe art without departing from the spirit of the invention.

Having thus described our invention, what we claim as new, and desire tosecure by Letters Patent is:
 1. A method of selectively electrochemicalmachining regions of a surface of a non-photolytically active workpiececomprising the following steps:placing the surface to be machined in anelectrolyte; placing a cathode in electrolyte; focusing an energy beamhaving an intensity of between about 10² W/cm² and 10⁶ W/cm² onto theworkpiece to locally heat the regions of the surface where preferentialmachining is sought; and establish an electric potential between theworkpiece serving as an anode and said cathode.
 2. The method of claim 1wherein said beam is laser generated.
 3. The method of claim 2 whereinsaid beam is modulated.
 4. The method of claim 2 wherein the surface tobe machined is not greater than 5 mil and is supported on a nonthermalconducting substrate.
 5. A method for electrochemical machining a regionof a surface of a non-photolytically active workpiece employing anelectrolyte and a cathode, the improvement comprising:directing a lightbeam having an intensity of greater than about 10² W/cm² to heat theregion of the surface to be machined to increase the machining rate. 6.A method for selectively electrochemical machining regions of a surfaceof a workpiece comprising the following steps:placing a cathode in anelectrolyte; focusing a modulated laser beam having an intensity ofbetween about 10² W/cm² and 10⁶ W/cm² onto a workpiece to locally heatthe regions of the surface where preferential machining is sought; andapplying an electrical potential between the workpiece which serves asan anode and the cathode, said electrical potential being pulsed insynchronism with said modulated beam.
 7. A method for chemical machiningregions of a surface by contacting the surface with an etching solution,the improvement comprising:directing a light beam having an intensity ofgreater than about 10² W/cm² to heat the region of the surface to bemachined to increase the etching rate.